The steps involved in the manufacture of an integrated circuit include the slicing of a semiconductor crystal into thin wafers, polishing of the wafers, preparation of masks or reticles, oxidation of the wafers, doping of the semiconductor material comprising the wafers, and processing the wafers through circuit forming steps such as photolithography, etching, exposure, etc. Once the circuits have been formed on a wafer, the individual circuits are probed to confirm that they are functional, and the wafer is then cut into chips and the individual chips packaged. At any one of these steps in the manufacturing process, it may be necessary to visually inspect the wafers for imperfections. In the past, the manufacturing steps, including the inspections, have typically been carried out in a clean room from which dust and other airborne particles have been substantially excluded.
Recently a system has been designed to permit the handling of semiconductor wafers both inside and outside of clean rooms. The system incorporates a standardized mechanical interface, and is commonly referred to by the acronym SMIF. The SMIF system includes dust-proof boxes for the transport and storage of the wafers, and is described in detail in U.S. Pat. No. 4,746,256, and U.S. patent application Ser. No. 050,952, filed May 15, 1987. In the apparatus described therein, a canopy is provided to enclose a dust-free environment in which are disposed a plurality of workstations. At least one of the stations serves as a port through which wafers stacked inside a SMIF container are conveyed through the canopy. Additional stations may include a microscope for inspecting the wafers, an ion deposition apparatus, photolithography apparatus, chemical baths for treating the wafer, etc. A centrally disposed robot manipulator is provided to handle the individual wafers, conveying them from station to station. Since the wafers are extremely delicate and likely to be damaged by a conventional clamping manipulator, the robot manipulator includes a vacuum pad that engages the lower surface of an individual wafer, enabling the wafer to be safely lifted and moved about inside the canopy.
In handling semiconductor wafers for inspection and during the process by which integrated circuits are manufactured from the wafers, it is sometimes necessary to center a wafer relative to a reference point and to rotate it to bring a discerning feature into a desired angular orientation. The discerning feature may comprise either a flat or a notch provided at one side of the wafer that serves as an alignment reference for subsequent steps in the integrated circuit manufacturing process. In the case of rectangular wafers, it may be necessary to position the edges in a particular orientation. The wafers are not centered or aligned in the SMIF containers. Due to the variations in the dimensions of the wafers, the tolerance and the dimensions of the SMIF containers, and the clearance between the edges of the wafers and sides of the containers, the deviation from concentricity of disc-shaped wafers stacked in the containers is typically on the order of plus or minus three millimeters. However, should the flat of a wafer happen to be aligned with and proximate to a side of the container, the error in concentricity may exceed 8 millimeters. The flat or other discerning feature on wafers stacked in SMIF containers tends to be randomly oriented.
Mechanical devices, including edge guides and drop cones, are available for centering an object or at least aligning it with a vertical surface; however, such prior art devices may damage a fragile semiconductor wafer. Furthermore, since wafers may have a range of diameters from 3 to 8 inches, it is desirable that the centration process be somewhat universal in application to at least a limited range of wafer sizes, thus obviating the need for a different centration station in the wafer handling apparatus for each size or shape of wafer. The centration station should be readily accessed by the manipulator robot so that once a wafer has been centered and angularly oriented as desired, it may be readily moved to another station for further processing or inspection. In addition, the accuracy of the centration process should be less than or equal to the translation precision of the manipulator, and thus sufficient to meet the requirements for positioning a wafer for inspection, or prealigning it for more critical manufacturing processes.